MLA

Whitman, Walt, 1819-1892. Lee, J. Fitzgerald. Box 2, folder 69. 1881 Dec 20. https://collections.library.yale.edu/catalog/2053703.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 2, folder 69"])(1881 Dec 20)[Lee, J. Fitzgerald]. https://collections.library.yale.edu/catalog/2053703.